This invention relates to measuring the level or depth of liquids using ultrasonic signals, such as for example, measuring the level of a body of water as part of a flow rate measurement.
It is known to use ultrasonic signals for measuring distances including the distance from a reference point above a body of water to the surface of the body of water. In a known class of such ultrasonic level measuring devices, ultrasonic pulses, each containing sound waves repeated at an ultrasonic frequency, are periodically transmitted and reflections or echoes of the ultrasonic pulses from the water are received. The time between transmitting an ultrasonic pulse and receiving its reflection is used to calculate the distance of the water from which the ultrasonic pulse was reflected. In such systems, the pulses are transmitted and received periodically at a predetermined ultrasonic carrier frequency and the time average of the reflections is used to more precisely determine distance.
In a prior art type of ultrasonic level measuring apparatus of this class, each time a pulse is transmitted, a series of reflections is received and all of the reflections are averaged. The average signal is utilized to determine a peak time of arrival and the peak time of arrival is compared with the time of transmission to determine the distance. This prior art system has the disadvantages of including noise and having a slow response time to changes in level when used as a water level sensing system.